Tires for passenger vehicles usually have a carcass reinforcement which in the great majority of cases today is radial; in this configuration the reinforcing elements of the carcass reinforcement are arranged to form an angle of greater than or equal to 80° with the circumferential direction (which amounts to saying that the reinforcing elements are either contained in a meridian plane—meaning a plane containing the axis of rotation—or form with this meridian plane an inclination of no more than 20°.
These tires usually have a crown reinforcement comprising a plurality of reinforcing elements embedded in an elastomer-based material. This crown reinforcement is covered radially outwardly by a rubber tread, and this tread has a surface, called the tread surface, which is designed to be in contact with the road as the tire rolls along.
To ensure an indispensable level of safety in rainy weather in particular, it is known practice to provide the tread with a plurality of grooves of generally circumferential (or longitudinal) orientation, and grooves of generally transverse orientation (parallel to or forming a small angle with the axis of rotation).
One problem with having longitudinal grooves is the generation of vibrations of the air moving along these grooves especially as they contact the road. These vibrations are the source of resonances which generate rolling noise.
Patent application publication JP-01-191734 examines this phenomenon and proposes reducing the noise generated by the air vibrating inside these grooves by forming within the thickness of the tread of a tire for a heavy vehicle, spherical cavities connected by a smaller-sectioned channel to a longitudinal groove. By this means it is possible to make a sort of resonator working on the principle of a Helmholtz resonator: such a cavity, because of its precisely determined volume, performs this function and reduces the effect of some of the frequencies of vibration of the air moving along the grooves as the grooves pass through the road contact patch. For each selected frequency, the volume of these cavities must be appropriate: the application to tires for a heavy vehicle is possible because, as a general rule, the tread elements are thicker than the tread elements of tires for passenger vehicles.
For a passenger vehicle, the tread is thin (that is, less than 10 mm), so it becomes essential to locate these resonators as far as possible from the tread surface of the tread and more particularly at a depth greater than the depth of the grooves, or at the very least at the depth of said grooves minus the legally required amount (indicated by the wear indicators which are pieces of rubber at the bottom of the grooves, the radially outward surface of which indicates the tread wear limit which should not be exceeded in order to maintain a satisfactory level of safety. While it would seem possible in the case of tires for heavy vehicles, as described in JP-01-191734, to mould and demould a tire with the mould elements moulding the hemispherical cavities, and these cavities communicating with the grooves, this becomes difficult with tires for passenger vehicles if only because of the great length of these cavities (the length being necessitated by the necessary volume).
Another problem with JP-01-191734 is that the large-volume spherical cavity can affect the way in which the rubber element containing it is compressed against the ground as the tire rolls along and in particular the distribution of the contact pressures. Also, the cavities of JP-01-191734 do not have a long enough life because after partial wear of the tread they become exposed on the tread surface, at which point the volume is no longer maintained and the resonator function is impaired.